Miniature freefall mechanism

ABSTRACT

A system for measuring differential gravity at two points is disclosed. In the illustrative embodiment, the system uses a pair of graspers which each repeatedly grasp, raise, and drop a test mass. The accelerations of the two free-falling test masses are monitored using optical interferometry. An output signal is provided that is based on a differential acceleration of the two test masses.

CROSS REFERENCE TO RELATED APPLICATIONS

The underlying concepts, but not necessarily the language, of thefollowing case is incorporated by reference: U.S. Pat. No. 5,892,151,issued 6 Apr. 1999. If there are any contradictions or inconsistenciesin language between this application and one or more of the cases thathave been incorporated by reference that might affect the interpretationof the claims in this case, the claims in this case should beinterpreted to be consistent with the language in this case.

FIELD OF THE INVENTION

The present invention relates to gravimeters in general, and, moreparticularly, to differential gravimeters.

BACKGROUND OF THE INVENTION

A differential gravimeter measures a gravity variation between twolocations. In addition, multiple differential gravimeters can be used todevelop a three-dimensional map of gravity. Such 3-D mapping has beenproposed in order to monitor fluid flow in-situ in subterraneanreservoirs, such as an oil field. In order to be used in an in-situunderground application, however, the gravity sensors of a gravimetermust be directly insertable into boreholes of the oil field. As aresult, the gravity sensors must be small and robust.

Since the differences in gravity across an oil field are typically veryslight, the gravimeter must be extremely sensitive; sensitivity below 1micro-Galileo is often necessary. Such extreme sensitivity, however,requires very high immunity to noise sources. Error can be introducedinto the output signal of a gravimeter from noise sources such aselectromagnetic interference, horizontal components in the accelerationof a free-falling mass, mechanical misalignment of sub-components,mechanical shock, and Coriolis forces that arise due to the rotation ofthe Earth.

Gravimeters have been developed that are based on the principle ofbalancing the weight of a fixed mass with forces from a normal orsuperconducting spring. Gravimeters such as these, however, have gravitysensors that are typically too large to be inserted into a borehole ofan oil well. They are also difficult to setup and calibrate. Inaddition, they are sensitive to environmental influences such astemperature and vibration.

More recently small gravimeters have been developed that include gravitysensors specifically designed for direct insertion into a borehole.These small gravimeters utilize piezoelectric launchers to verticallylaunch a pair of test masses upward so that they can subsequentlyfree-fall downward. An interferometer arrangement is used to monitor theacceleration of their falling masses after each reaches its apex. Inaddition to some of the drawbacks of other prior-art gravimeters,however, noise due to shock and vibration caused by their piezoelectriclaunchers limits the sensitivity of these gravimeters.

There exists a need, therefore, for a gravimeter that avoids ormitigates some or all of the problems associated with prior-artgravimeters.

SUMMARY OF THE INVENTION

The present invention provides a differential gravity measurementsystem. Some embodiments of the present invention are particularlywell-suited for monitoring oil flow in subterranean oil fields. Inparticular, the illustrative embodiment of the present invention uses apair of optically-interrogated, free-falling test masses in a Michelsoninterferometer arrangement to provide a highly sensitive measurement ofthe difference in gravity at two locations.

In the illustrative embodiment, the gravimeter comprises aninterferometer and two gravity sensors that are optically interrogatedas part of an interferometer arrangement. Each gravity sensor comprisesa mass dropper, which includes a grasper and a test mass, and a gimbalfor orientating of the mass dropper to vertical. Each grasper ismechanically-coupled to an actuator that moves it from a first position,wherein the grasper passively grasps the test mass, to a secondposition, wherein the grasper passively releases the test mass therebyallowing it to free-fall. A processor synchronizes the release of thetest masses by their respective graspers. An optical beam is reflectedoff of each of the two falling test masses as they fall. These twooptical paths compose the reference and test legs of the interferometerarrangement. The output signal of the interferometer is based on adifference in the path lengths of these optical beams; therefore, theoutput signal of the interferometer is a function of the difference inthe local gravity that acts on each test mass.

Some embodiments of the invention comprise a tilt sensor that provides afeedback signal used to minimize the tilt of the mass dropper. Someembodiments of the invention comprise a rotation sensor to provide asignal based on a rotation of the test mass as it falls. This signal isused to provide a post-drop correction for mitigating the effects on thegravity sensor due to component misalignment, residual tilt of the massdropper, effects from the rotation of the earth, and the like.

An embodiment of the present invention comprises: (1) a mass droppercomprising; (i) a first test mass; (ii) a grasper, wherein the graspergrasps the first test mass when in a first position, and wherein thegrasper releases the first test mass when in a second position; and(iii) an actuator for moving the grasper on a path that includes thefirst position and the second position; and a sensor for providing afirst signal based on an orientation of the mass dropper with respect tovertical; and (2) a gimbal for controlling the orientation of the massdropper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of details of an oil field fluid flowmeasurement system in accordance with an illustrative embodiment of thepresent invention.

FIG. 2 depicts a schematic diagram of details of a gravimeter inaccordance with the illustrative embodiment of the present invention.

FIG. 3 depicts a schematic diagram of details of a gravity sensor inaccordance with the illustrative embodiment of the present invention.

FIG. 4 depicts a cross-sectional diagram of details of a mass dropper,prior to grasping a test mass, in accordance with the illustrativeembodiment of the present invention.

FIG. 5 depicts a cross-sectional view of a mass dropper, after graspinga test mass, in accordance with the illustrative embodiment of thepresent invention.

FIG. 6 depicts a cross-sectional view of a mass dropper, after releaseof a test mass, in accordance with the illustrative embodiment of thepresent invention.

FIG. 7 depicts a cross-sectional view of an actuator in accordance withthe illustrative embodiment of the present invention.

DETAILED DESCRIPTION

Full-field differential gravity monitoring for modeling the fluiddistribution in an oil field can be achieved by means of mapping therelative gravity across the area of an oil field. An effective methodfor mapping the relative gravity is the application of doubledifferences to detect gravity changes.

In this method the differential gravity is measured at a plurality ofpoints, referenced to a base location. These differential gravitymeasurements are used to develop a full-field gravity image versus time,beginning with an initial image at time, t=0. Changes in oildistribution in the oil field can be determined by comparing subsequentimages either by referencing each to the initial image, or by comparingsequential images. In order to minimize random errors, each image mayinclude an average of tens or hundreds of individual measurementsbetween set of two points. Each set of points, comprising thedifferential gravity measurement between two points, can be obtained bymeans of a differential gravimeter in accordance with the presentinvention.

FIG. 1 depicts a schematic diagram of details of an oil field fluid flowmeasurement system in accordance with an illustrative embodiment of thepresent invention. Measurement system 100 comprises differentialgravimeter 102, and oil wells 104-1 and 104-2.

Gravimeter 102 is a differential gravity measurement system formonitoring fluid movement in the oil field in order to maximizeproduction yield. Gravimeter 102 comprises gravity sensors 106-1 and106-2, cables 108-1 and 108-2, and controller 110. In order to monitorfluid flow in the oil field, the multiple gravity sensors are inserteddirectly into the boreholes of oil wells that are distributed around theoil field. Each pair of sensors provides a differential gravity readingbetween their locations. In total, the sensors provide a measure of thegravity gradient in the oil field, and thus provide an indication of itsoil distribution. Gravimeter 102 is described in more detail below andwith respect to FIG. 2.

Each of gravity sensors 106-1 and 106-2 provide an optical signal thatis reflected from a free-falling test mass contained within it. Theoptical signals are carried to controller 110 via optical fiberscontained in cables 108-1 and 108-2. A change in the relative phase ofthese two optical signals denotes a difference in the accelerations ofthe free-falling test masses. This phase information, therefore, denotesa difference in the local gravity experienced by each gravity sensor.Gravity sensors 106-1 and 106-2 are described in detail below and withrespect to FIGS. 3-7.

Controller 110 is a processor/controller for: (1) supplying opticalsignals to gravity sensors 106-1 and 106-2; (2) detecting opticalsignals reflected from gravity sensors 106-1 and 106-2; and (3)generating an output based on the phase difference between the opticalsignals reflected from gravity sensors 106-1 and 106-2. Controller 110is described in more detail below and with respect to FIG. 2.

FIG. 2 depicts a schematic diagram of details of a gravimeter inaccordance with the illustrative embodiment of the present invention.Gravimeter 102 comprises gravity sensors 106-1 and 106-2, and controller110. Controller 110 comprises a general purpose signal processor as wellas optical components that, together with gravity sensors 106-1 and106-2, form an interferometer for providing differential phaseinformation used by controller 110 to generate its output. Specifically,controller 110 comprises source 202, beamsplitter 204, detector 210, andprocessor 214.

Source 202 is a source of an optical signal comprising substantiallymonochromatic light. This light is launched into optical fiber 206,which conveys the light to beamsplitter 204. It will be clear to thoseskilled in the art how to make and use source 202.

Beamsplitter 204 is a conventional beamsplitter that is positioned asthe central component in a Michelson interferometer configuration.Beamsplitter 204 splits optical energy received from optical fiber 206into two optical signals (which are in-phase), and launches theseoptical signals into optical fibers 208-1 and 208-2. Beamsplitter 204also receives optical signals reflected from gravity sensors 106-1 and106-2. The reflected signals are combined and launched onto opticalfiber 212. It will be clear to those skilled in the art how to make anduse beamsplitter 204.

Detector 210 is a conventional photodetector which generates anelectrical signal based on the intensity of light received from opticalfiber 212. Since the signal received by detector 210 is the combinedreflected optical signals from gravity sensors 106-1 and 106-2, itselectrical output exhibits any effects of the interference of thesereflected optical signals. This interference is an indication of anydifference in the accelerations of free-falling masses in gravitysensors 106-1 and 106-2, as will be explained below and with respect toFIGS. 4-6. A difference in these accelerations is a function of adifference in gravity between the locations of gravity sensors 106-1 and106-2. It will be clear to those skilled in the art how to make and usedetector 210.

Processor 214 is a general purpose processor that: (1) generates anoutput signal based on an electrical signal received from detector 210;(2) provides orientation control signals to gravity sensors 106-1 and106-2; and (3) synchronizes the release of the test masses in gravitysensors 106-1 and 106-2. Although the illustrative embodiment comprisesa processor that interfaces with only one pair of gravity sensors, itwill be clear to those skilled in the art, after reading thisspecification, how to make and use alternative embodiments of thepresent invention wherein processor 214 interfaces with multiple pairsof gravity sensors.

Electrical cables 216-1 and 216-2 are conventional control cables forconveying electrical signals between processor 214 and gravity sensors106-1 and 106-2, respectively. Electrical cable 216-1 is bundled withoptical fiber 208-1 in cable 108-1. In similar fashion, electrical cable216-2 is bundled with optical fiber 208-2 in cable 108-2.

FIG. 3 depicts a schematic diagram of details of a gravity sensor inaccordance with the illustrative embodiment of the present invention.Gravity sensor 106 is representative of either of gravity sensors 106-1and 106-2. Gravity sensor 106 comprises housing 302, mass dropper 304,gimbal frame 306, tilt sensor 308, gimbal actuators 310, ballast 312,and ferrule 314.

Housing 302 is a rigid housing for protecting gravity sensor 106 whileit is submerged in the harsh environment of an oil well. It will beclear to those skilled in the art, after reading this specification, howto make and use housing 302.

Mass dropper 304 is a system for dropping a test mass so that itsfree-fall acceleration can be determined. Optical signal 316, which istransmitted to and reflected from the test mass, is coupled to opticalfiber 208 via ferrule 314. Mass dropper 304 will be described below andwith respect to FIGS. 4-6.

Gimbal frame 306 is a frame of rigid material that is configured toenable mass dropper 304 to rotate about two orthogonal axes relative tohousing 302. Gimbal frame 306 is connected to mass dropper 304 via afirst set of gimbal actuators 310, such that mass dropper 304 can rotateabout the Y-axis (as shown in FIG. 3) with respect to gimbal frame 306.Gimbal frame 306 is connected to housing 302 via a second set of gimbalactuators 310 which are oriented orthogonally with respect to the firstset of gimbal actuators 310. The second set of gimbal actuators enablethe rotation of gimbal frame 306 about the X-axis (as shown in FIG. 3)with respect to housing 302. As a result, the first and second set ofgimbal actuators 310 enable the rotation of mass dropper 304 about twoorthogonal axes with respect to housing 302.

Tilt sensor 308 is a conventional electrolytic tilt sensor that providesan electrical signal based on the tilt of mass dropper with respect tovertical. It will be clear to those skilled in the art, after readingthis specification, how to make and use tilt sensor 308. Although theillustrative embodiment comprises an electrolytic tilt sensor, it willbe clear to those skilled in the art, after reading this specification,how to make and use alternative embodiments of the present inventionwherein tilt sensor 308 comprises other types of tilt sensors havingsufficient accuracy. Tilt sensors suitable for use in tilt sensor 308include, without limitation, accelerometers, MEMS accelerometers,mercury-based tilt switches, rotary encoders, and inertial sensors.

Gimbal actuators 310 are piezoelectric ultrasonic ring motors, which arecapable of high-precision rotation. Although in the illustrativeembodiment gimbal actuators 310 comprise ultrasonic ring motors, it willbe clear to those skilled in the art, after reading this specification,how to make and use alternative embodiments of the present inventionwherein gimbal actuators 310 comprise any type of ring motor havingsufficient accuracy. Gimbal actuators 310 also comprise slip-ringelectrical contacts for providing electrical connectivity betweencomponents in mass dropper 304 and electrical cable 216.

Deviation of the orientation of mass dropper 304 from vertical resultsin errors caused by horizontal components in the gravity sensor outputsignal. It is desirable, therefore, that the orientation of mass dropper304 be as close to vertical as possible. For the purposes of thisspecification, including the appended claims, “vertical” means thatorientation that causes a test mass in a mass dropper to fall with nohorizontal displacement component (i.e., wherein the mass dropper is“plumb”), and “tilt” means a deviation from vertical. Tilt sensor 308,processor 214, and gimbal actuators 310 constitute a feedback system forminimizing the tilt of mass dropper 304. Processor 214 provides controlsignals to gimbal actuators 310 to minimize the output of tilt sensor308.

Ballast 312 is a solid mass of highly-dense material that is located onthe underside of house 302. Ballast 312 alters the weight distributionfor housing 302 and causes gravity sensor 106 to orient itself in anearly vertical orientation. The presence of ballast 312, therefore,reduces the amount of travel required of the gimbal actuators 310 tominimize the tilt the gravity sensor. As a result, optical port 318maintains a rough alignment with ferrule 314 to allow the passage offree-space optical beam 316 through gimbal frame 304. Since ballast 312serves to keep optical port 318 roughly aligned with ferrule 314,optical fiber 208 needs only a small amount of slack to accommodate therelative motion of mass dropper 306, gimbal frame 304, and housing 302required to put mass dropper 306 in vertical orientation.

Although in the illustrative embodiment, housing 302 comprises ballastthat results in rough alignment of optical port 316 and ferrule 314,some alternative embodiments do not comprise ballast 312. In somealternative embodiments, optical fiber 108 includes a loose coil ofoptical fiber to accommodate large rotations of mass dropper 306 withrespect to housing 302. In some alternative embodiments, free spaceoptical signal 316 is routed from ferrule 314 through gimbal frame 304and dropper frame 402 via a plurality of mirrors located on the innersurface of gimbal frame 304. It will be clear to those skilled in theart, after reading this specification, how to make and use alternativeembodiments of the present invention wherein housing 302 does notcomprise ballast.

Ferrule 314 is a conventional fiber optic ferrule for transmitting andreceiving optical signal 316 to/from mass dropper 306. Ferrule 314includes a lens for efficiently coupling free-space optical signal 316into and out of optical fiber 208. Ferrule 314 also includes a facethaving an integrated turning element, such as a mirror, wedge, prism,and the like. In some alternative embodiments, ferrule 314 is notintegrated with the turning element. In some alternative embodiments,ferrule 314 is oriented to launch optical signal 316 directly at massdropper 306 without the need for a turning element.

FIGS. 4-6 depict a cross-sectional diagram of details of a mass dropper:(1) prior to grasping a test mass; (2) after grasping a test mass; and(3) after releasing a test mass, in accordance with the illustrativeembodiment of the present invention. Mass dropper 306 comprises dropperframe 402, grasper 404, test mass 406, actuator 412, wedge 416, and seat418.

Dropper frame 402 is a frame of rigid material that is configured toenable mass dropper 304 to rotate about an axis with respect to gimbalframe 304. Dropper frame 402 is connected to gimbal frame 304 via gimbalactuators 310. Dropper frame 402 also provides a stable platform forpositioning grasper 404 and seat 422 such that grasper 404 drops testmass 406 directly into seat 418 when mass dropper 306 is verticallyoriented. Dropper frame 402 comprises optical port 422, which enablesoptical signal 318 to interrogate test mass 406 throughout its entirerange of travel.

Grasper 404 is a rigid platform having a plurality of tangs 408projecting from one face. Together, tangs 408 compose a pincer forpassively engaging catch 410 of to grasp test mass 406. Tangs 408 aremade of a resilient material, and thus generate a restoring force whenforced apart by catch 410. When grasper 404 is moved into engagementwith test mass 406 by actuator 412, the top of catch 410 forces tangs408 to separate. As grasper 404 is moved into further engagement withtest mass 406, tangs 408 spread over catch 410. The restoring forcegenerated within tangs 408 causes grasper 404 to grasp test mass 406.Tangs 408 exert a substantially uniformly-distributed force on the outersurface of catch 410. Uniform distribution of the grasping force oncatch 410 results in a smooth release of test mass 406 when grasper 404releases it. As a result, grasper 404 does not induce substantialrotation of test mass 406 as it drops. Although in the illustrativeembodiment grasper 404 comprises four tangs, it will be clear to thoseskilled in the art, after reading this specification, how to make anduse alternative embodiments of the present invention wherein grasper 404comprises any number of tangs.

Test mass 406 is a circular mass having a shaped bottom surface formating with seat 418. Test mass 406 comprises catch 410 andretroreflector 420. Its shaped bottom surface ensures that test mass 406will locate in seat 418 in substantially the same orientation each andevery time that test mass 406 is dropped. Retroreflector 420 reflectsfree-space optical signal 316 on a return path parallel to, andpreferably coincident with, its path to test mass 406 from ferrule 314.

Actuator 412 is a linear actuator that is affixed to grasper 404.Actuator 412 moves grasper 404 along shaft 414 from a first position, inwhich grasper 404 grasps catch 410, to a second position, in whichgrasper 404 releases catch 410. Actuator 412 comprises a conventionalMEMS-based inch-worm actuator, which is capable of high-precision motionalong shaft 414. Actuator 412 will be discussed in more detail below andwith respect to FIG. 7. Although the illustrative embodiment comprisesan inch-worm linear actuator, it will be clear to those skilled in theart, after reading this specification, how to make and use alternativeembodiments of the present invention wherein actuator 412 comprisesother linear actuators. Suitable actuators for use in actuator 412include, without limitation, solenoids, electromagnetic linear motors,lead screw systems, and the like.

Shaft 414 is a steel shaft suitable for use with actuator 412. Shaft 414includes two flats (not shown) on opposite sides. These flats provide alarger contact surface for actuator 412. In some alternativeembodiments, shaft 414 does not include flats.

Wedge 416 is a conical projection attached to the free end of shaft 414.The shape of wedge 416 is suitable for smoothly engaging tangs 408thereby causing their separation. The diameter of wedge 416 is slightlylarger than the diameter of catch 410 to ensure that tangs 408 releasecatch 410 when they are sufficiently engaged with wedge 416.

Seat 418 is a recess in dropper frame 402. Seat 418 is shaped to accepttest mass 406 such that test mass 406 locates in substantially the sameposition and orientation after each drop by grasper 404. Seat 418further comprises optical port 422, which provides access toretroreflector 420 for optical beam 316.

Errors in the output of a gravity sensor can arise from sources such astilt of the mass dropper, component misalignments, and Coriolis forcescaused by the Earth's rotation. In order to mitigate some of the effectsof at least some error sources, a post-drop correction can be appliedbased on the measured rotation of test mass 406 as it free-falls. Massdropper 304, therefore, includes an optional rotation sensor, whichcommunicates with processor 214 via electrical cable 216.

Rotation sensor 424 comprises laser diodes 426-1 and 426-2 andposition-sensitive detectors (PSDs) 430-1 and 430-2. Laser diode 426-1reflects light beam 428-1 off of test mass 406 to PSD 430-1. In similarfashion, laser diode 426-2 reflects light beam 428-2 off of test mass406 to PSD 430-2. If test mass 406 has not rotated with respect to massdropper 304 (such as when located in seat 418), PSDs 430-1 and 430-2receive light beams 428-1 and 428-2 at the same elevation. The outputvoltage of each PSD, therefore, will be the same. If, during itsfree-fall, test mass 406 has rotated with respect to mass dropper 304,PSDs 430-1 and 430-2 will receive light beams 428-1 and 428-2 atdifferent elevations. As a result, the outputs of PSDs 430-1 and 430-2will differ as a function of the degree of rotation of test mass 406.Although in the illustrative embodiment rotation sensor 424 comprisesposition-sensitive detectors, it will be clear to those skilled in theart, after reading this specification, how to make and use alternativeembodiments of the present invention wherein rotation sensor 424comprises any detector whose output is a function of received beamlocation, such as charge-coupled-device (CCD) strip detectors,photodetector arrays, and the like. It will also be clear to thoseskilled in the art, after reading this specification, how to make anduse alternative embodiments of the present invention wherein rotationsensor 424 comprises sources of optical energy other than laser diodes.Since the output of each of PSD 430-1 and 430-2 is indicative of theposition of mass 406 along its direction of travel, in some embodiments,one or both of PSD 430-1 and 430-2 is used to indicate when mass 406nears seat 418. In other words, in such embodiments, rotation sensor 424is also a proximity sensor.

FIG. 5 depicts a cross-sectional view of a mass dropper, after graspinga test mass, in accordance with the illustrative embodiment of thepresent invention. Once grasper 404 has grasped test mass 406, actuator412 moves grasper 404 upward toward wedge 416. Tangs 408 are designed toexert evenly-distributed pressure on catch 410 so that no rotation oftest mass 406 occurs as it is lifted by grasper 404.

FIG. 6 depicts a cross-sectional view of a mass dropper, after releaseof a test mass, in accordance with the illustrative embodiment of thepresent invention. Grasper 404 has engaged wedge 416, which forces tangs408 to separate. The separation of tangs 408 causes grasper 404 tosmoothly release test mass 404, which allows test mass 404 to begin itsfree-fall without induced horizontal velocity components or rotation.Free-space optical beam 316 interrogates test mass 406 during itsfree-fall via retroreflector 420. Reflected optical signal 316 iscoupled into optical fiber 208 via ferrule 314. As test mass 406 falls,rotation sensor 424 monitors its rotation via light beams 428-1 and428-2.

FIG. 7 depicts a cross-sectional view of an actuator in accordance withthe illustrative embodiment of the present invention. Actuator 412comprises body 502, couplings 504-1 and 504-2 (referred to,collectively, as couplings 504), elongation elements 506-1 and 506-2(referred to, collectively, as elongation elements 506), upper contactpads 508-1 and 508-2 (referred to, collectively, as upper pads 508), andlower contact pads 510-1 and 510-1 (referred to, collectively, as lowerpads 510).

Body 502 is a ring of rigid material and couplings 504 are bars of rigidmaterial that couple grasper 404 to the elongation elements 506.Suitable materials for body 502 and couplings 504 include, withoutlimitation, metals, ceramics, plastics, and carbon-based materials.

Elongation elements 506 are bars of piezoelectric material that elongatealong their longitudinal axis energized with sufficient voltage.

Upper pads 508 and lower pads 510 are pads of piezoelectric materialwhose contact surface has a coefficient of friction with outer surfaceof shaft 414 suitable for clamping shaft 414 without slipping. Whenenergized with sufficient voltage, upper pads 508 clamp shaft 414. Insimilar fashion, when energized with sufficient voltage, lower pads 510clamp shaft 414.

Actuator 412 is capable of high-precision motion along shaft 414.Actuator 412 moves upward along shaft 414 by means of a voltage cyclecomprising the steps: (1) energizing lower pads 510; (2) de-energizingupper pads 508; (3) energizing elongation elements 506 to increase theseparation between lower pads 510 and upper pads 508; (4) energizingupper pads 508; (5) de-energizing lower pads 510; and (6) de-energizingelongation element 506 to reduce the separation between lower pads 510and upper pads 508. Motion downward along shaft 414 is accomplished insimilar fashion with the appropriate change in the sequence of stepsabove.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order toprovide a thorough description and understanding of the illustrativeembodiments of the present invention. Those skilled in the art willrecognize, however, that the invention can be practiced without one ormore of those details, or with other methods, materials, components,etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1. An apparatus comprising: a mass dropper comprising; (i) a first testmass; (ii) a grasper, wherein said grasper passively grasps said firsttest mass when in a first position, and wherein said grasper releasessaid first test mass when in a second position; and (iii) an actuatorfor moving said grasper on a path that includes said first position andsaid second position; a sensor for providing a first signal based on anorientation of said mass dropper with respect to vertical; and a gimbalfor controlling the orientation of said mass dropper.
 2. The apparatusof claim 1 further comprising an optical system for providing an outputsignal based on the acceleration of said first test mass after itsrelease.
 3. The apparatus of claim 2 wherein said output signal is basedon the relation of: (1) the phase of a first optical signal reflectedfrom said first test mass and (2) the phase of a second optical signalreflected from a second test mass.
 4. The apparatus of claim 3 furthercomprising a synchronizer for substantially synchronizing the release ofsaid first test mass and the release of said second test mass.
 5. Theapparatus of claim 1 wherein said mass dropper further comprises aproximity sensor for providing a second signal based on the position ofsaid actuator on said path.
 6. The apparatus of claim 1 wherein saidmass dropper further comprises a wedge, and wherein said wedge causessaid grasper to release said first test mass when said grasper is insaid second position.
 7. The apparatus of claim 1 wherein said actuatorcomprises a linear motor.
 8. The apparatus of claim 1 wherein saidactuator comprises an inchworm motor.
 9. The apparatus of claim 1wherein said first mass dropper further comprises a rotation sensor forsensing a rotation of said first test mass.
 10. An apparatus comprising:(1) a first mass dropper, wherein said first mass dropper comprises; (i)a first test mass; (ii) a first grasper, wherein said first graspergrasps said first test mass when said first grasper is in a firstposition, and wherein said first grasper does not grasp said first testmass when said first grasper is in a second position; (iii) a firstactuator for moving said first grasper on a first path that includessaid first position and said second position; and (2) a first gimbal fororienting said first mass dropper in a substantially verticalorientation; (3) a second mass dropper, wherein said second mass droppercomprises; (i) a second test mass; (ii) a second grasper, wherein saidsecond grasper grasps said second test mass when said second grasper isin a third position, and wherein said second grasper does not grasp saidsecond test mass when said second grasper is in a fourth position; (iii)a second actuator for moving said second grasper on a second path thatincludes said third position and said fourth position; and (4) a secondgimbal for orienting said second mass dropper in a substantiallyvertical orientation; and (5) an optical system, wherein said opticalsystem provides an output signal based on the phases of a first opticalsignal and a second optical signal, wherein said first optical signal isreflected from said first test mass and said second optical signal isreflected from said second test mass.
 11. The apparatus of claim 10further comprising a first wedge and a second wedge, wherein said firstwedge causes said first grasper to release said first test mass whensaid first grasper is moved to said second position, and wherein saidsecond wedge causes said second grasper to release said second test masswhen said second grasper is moved to said fourth position.
 12. Theapparatus of claim 11 further comprising a synchronizer, wherein saidsynchronizer synchronizes the release of said first test mass and therelease of said second test mass.
 13. The apparatus of claim 10 whereinsaid first grasper comprises a plurality of tangs, and wherein saidplurality of tangs engages and grasps said first test mass when saidfirst grasper moves to said first position, and further wherein saidfirst wedge separates said tangs to release said first test mass whensaid first grasper moves to said second position.
 14. The apparatus ofclaim 10 further comprising a first sensor, wherein said first sensorprovides a signal based on an orientation of said first mass dropperwith respect to vertical.
 15. The apparatus of claim 14 wherein saidfirst sensor comprises an accelerometer.
 16. The apparatus of claim 14wherein said first sensor comprises an electrolytic tilt sensor.
 17. Theapparatus of claim 10 wherein said first actuator comprises a linearactuator.
 18. The apparatus of claim 10 wherein said first actuatorcomprises an inchworm motor.
 19. The apparatus of claim 10 wherein saidfirst actuator comprises a lead screw.
 20. The apparatus of claim 10wherein said first mass dropper further comprises a rotation sensor forsensing a rotation of said first test mass.